Embodiments herein generally relate to using tone reproduction curves to maintain optimal print engine performance.
As described in U.S. Patent Publication 2004/0165199 (incorporated herein by reference) an imaging device in the form of a printer or copier typically creates images using combinations of four colors of marking agents or colorants, such as cyan, magenta, yellow and black (CMYK). The images are created based on image data which assigns at least one of the four colors and a numerical color intensity or input color value to each picture element or pixel in the image.
A problem is that, due to manufacturing variations, different imaging devices can output different intensities of color based on identical image data. The density of the toner laid down on the print medium determines the color intensity. The denser or thicker the toner is laid down on a white print medium such as paper, the less white is visible through the toner on the paper. Consequently, the denser the toner, the less the lightness of the toner color, and the greater the intensity of the toner color.
Because there is such variation in toner density laid down by different imaging devices based on identical image data, color intensities that are output by some imaging devices can be outside of an acceptable range. Thus, in order to ensure that each imaging device outputs color intensities that closely correspond to the color intensities specified by the image data, each imaging device should be individually calibrated to output appropriate color intensities.
One approach in calibrating an imaging device is to use a tone reproduction curve (TRC), which is a modeled mathematical relationship between the input color values and the colorant amounts that must be sent to the imaging device in order to produce the intensities specified by the input color values. The TRC is a continuous curve on a plot of input color values versus output colorant values that represents a best fit of the discrete data points matching each input color value with an output colorant value that, when rendered on the given device, produces the intensity specified by the input color value.
As described in U.S. Pat. No. 6,694,109 (incorporated herein by reference) in the case of printing devices such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge retentive surface or photoreceptor on which the electrostatic latent image is formed and subsequently developed by causing toner particles to adhere to areas thereof that are charged in a particular way. In such a case, an optical device, often referred to as a densitometer, for determining the density of toner on the test patch is disposed along the path of the photoreceptor directly downstream of the development unit. There is typically a process within the operating system of the printer to periodically create test patches of the desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to change or discharge as necessary the surface at the location to a predetermined extent.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere to the test patch electrostatically. The denser the toner on the test patch, the darker the test patch will appear in optical testing. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor and the light absorption of the test patch is tested. The density of toner on the patch varies in direct relationship to the percentage of light absorbed by the test patch.
Test patches that are used to measure the deposition of toner on paper to measure and control the tone reproduction curve (TRC) are traditionally printed on inter-document zones of the photoreceptor belts or drums. Generally, each patch is a small square that is printed as a uniform solid halftone or background area. This practice enables the sensor to read values on the TRC for each test patch.
A processor of the imaging device calculates a separate TRC for each of the colors or separations of the imaging device. The TRCs are used to calibrate the imaging device. More particularly, once such TRCs are established for an imaging device, the TRCs can be used to correlate input color values with imaging device output image colorant values or color intensities. In addition, a multi-dimensional look up table or LUT is often calculated to account for interactions among the colorants and to accommodate different input color spaces, for example, CIELAB or sRGB. Numerous techniques exist in the prior art for deriving LUTs and TRCs for printer calibration and characterization.
Each of the test patches is formed with a different combination of a colorant (C, M, Y or K) and a numeric input color value. The input color value specifies the desired colorant density, and consequently, the desired output lightness color value or color intensity. The density of the colorant on the test patches varies as a function of the input color value. Thus, optically measuring the density of the patches provides an indication of the input color values with which the patches were made. The denser the colorant on the test patch, the more light will be absorbed by the colorant, and the less light will be reflected back to the optical color-measuring device. This indicates a greater intensity provided by the colorant. The printed test patches are moved past the color-measuring device, and the light absorption of the test patch is measured.
The measurements of the test patches and the input color values used to construct them are used to calculate the TRC. The accuracy of the TRC increases with the number of data points that it is based upon. Measurement error drops by a factor of the square root of the number of measurements. However, from a cost viewpoint, it is desirable to minimize the number of test patches that are printed, since printing test patches consumes a significant amount of colorant.